Does this come with the mattress?
No, mattresses are not included with the bed.
I'm confused with the listed dimensions. You list: Twin 80"L x 42"W x 39"H and Twin XL 85"L x 42"W x 39"H. I have a Twin xl mattress that is 80"x38"....which frame should I get?
If you have a Twin XL mattress, you need the Twin XL frame. The dimensions listed account for the headboard and footboard, but the area for the mattress is the standard 80".
What is the weight limit on the half bunkbed's?
The half bunk beds are tested to 250 lbs. That's not necessarily the capacity, but that is the weight they have been tested to hold.